Solvent extraction method



March 19, 1963 c, McK'INNlS 3,082,270

SOLVENT EXTRACTION METHOD Filed July 31, 1961 woA/Aeo/wl 7 65 2 5; 440/144 77:5

401/5005 XTEACT A445 SOLVE/VT ,4QUEOU5 A4455 INVENTOR. ART :3 44 K/A/M/S BY )JLN W United States Patent nee 3,082,270 Patented Mar. 19, 1963 3,082,270 SOLVENT EXTRACTION METHOD Art C. McKinnis, North Long Beach, Calif, assignor to Union Oil Company of California, Los Angeles, Cairn, a corporation of California Filed July 31, 1961, Ser. No. 128,244 Claims. (Cl. 260-674) This invention relates to. a solvent extraction method for separating hydrocarbons of greater aromaticity from hydrocarbons of lesser aromaticity in admixture therewith. More specifically, the invention relates to such a method in which mixtures of ammonia and/ or amines and thiocyanic and/ or cyanic acid are employed as selective solvents for the hydrocarbons of greater aromaticity. The invention has particular utility for the recovery of diaromatic hydrocarbons, such as naphthalene or the like, from mixtures thereof with monoaromatic hydrocarbons of substantially equivalent boiling points, such as alkyl benzenes or the like.

There are a number of known solvent extraction procedures for isolating various components of hydrocarbon mixtures and numerous materials have been proposed for use as selective solvents in such procedures, typically representative of which are sulfur dioxide, furfural, diethylene glycol, nitriles, organic bases, etc. Such solvent extraction procedures have been attempted with varying degrees of success on mixtures of aromatic and nonaromatic hydrocarbons for purposes of extracting all of the aromatics therefrom, but heretofore it has not been possible to bring about any kind of effective fractional separation of the aromatic components from each other by solvent extraction means.

1 have now discovered an improved method of solvent extraction for use on hydrocarbon mixtures whereby aro matic components of differing degrees of aromaticity can be economically and effectively fractionated.

There are many hydrocarbon mixtures, as, for example, various petroleum processing fractions, which contain substantial proportions of diaromatic hydrocarbons such as naphthalene and its alkyl and polyalkyl derivatives, and also monoaromatic hydrocarbons which can be monocyclic, such as alkyl benzenes, and/or bicyclic, such as alkyl t'etralins, alkyl indanes, alkyl indenes and the like which boil within the same boiling range as the diaromatics. The boiling range of the diaromatics in hydrocarbon mixtures of this type is typically from about 400 to about 450 F. A specific example of such a hydrocarbon mixture is the heavy reformate fraction obtained in the catalytic reforming of naphthenes. This fraction normally boils above about 400 F. and contains from about 40 to about 80 percent by weight naphthalene and methyl naphthalenes, the remainder being largely made up of monoaromatic compounds such as alkyl benzenes, tetralins, indanes, indenes, and the like, or of such monoaromatics plus a significant proportion of nonaromatics such as naphthenes, parafiins, etc. Many other fractions obtained in the practice of petroleum fractionating and conversion operations, such as catalytic cracking, thermal cracking, catalytic reforming, catalytic cycle oil, etc. operations, also fit the above described category.

Hydrocarbon mixtures such as those described are difiicult of separation into their componential fractions, i.e., fractions containing separate or like components Within the diaromatic, monoaromatic, etc., categories by conventional fractionation means. For one thing, the like boiling point ranges of the diaromatics and monoaromatics normally found in such mixtures precludes the possibility of getting effective separation between these two classes of materials by fractional distillation techniques. The present invention comprises a method of solvent extraction empl-oying a particular type of amine-acid solvent by means of which componential segregation of aromatics is readily and economically feasible.

In addition to the problem of separating like boiling aromatics of differing degrees of aromaticity, there is a parallel problem of separating aromatics from nonaromatics of like boiling ranges. There are many such mixtures, exemplary of which are those petroleum fractions known to contain monoaromatics such as polyalkyl benzenes boiling within the range from about 400 to about 450 F. as well as nonaromatics such as paratfins and naphthenes of the same boiling range. Here again, as in the case of the like boiling aromatics, fractional distillation fails as a practical means of accomplishing the desired separation and the solvent extraction method of this invention furnishes a simple and practical solution to the problem. I

' It is thus a principal object of this invention to provide an improved solvent extraction method by means of which componential fractions of various aromaticity levels can be segregated from mixtures of such aromatic compounds.

It is another object of the invention to provide a solvent extraction method for readily and economically separating aromatic compounds from like boiling nonar omatic compounds in admixture therewith.

A more specific object of the invention is to provide an economical solvent extraction method by means of which diaromatic compounds are readily separable from like boiling mon-oaromatic compounds in admixture therewith. Other objects and advantages of the invention will be apparent from the complete description thereof which follows.

The degrees of aromaticity of organic compounds of roughly equivalent boiling points depend upon the number of aromatic rings (benzene nuclei) in their respective molecules, the higher the number of such rings the greater thearomaticity of a given compound. Thus diaromatic compounds, those having two aromatic rings per molecule, are considered to have a greater degree of aromaticity'than the monoaromatic compounds which have molecular structures containing only one such ring. It makes little difference, insofar as degree of aromaticity is concerned, whether polyarom-atic compounds have individual or condensed ring systems. Thus, diaromatics of individual ring systems such as hiphenyl, diphenylmethane, etc., are considered to have roughly the same degree of aromaticity as the dinuclear aromatics (diaromatics having condensed ring systems), such as naphthalene, etc., at least insofar as this invention is concerned.

The method of this invention is not limited to the treatment of organic mixtures in which the aromatics of highest possible degree of aromaticity are diaromatics, and mixtures containing higher polyaromatic compounds are also amenable to separation by said method. For example, it is within the scope of the invention to subject hydrocarbon mixtures containing triaromatic compounds, such as anthracene and phenanthrene, and diaromatic compounds of roughly the same boiling range to solvent extraction as taught herein, to separate the triaromatics from the diaromatics.

The method of this invention is not limited in application to the treatment of organic mixtures having clearly obvious differences in the degree of aromaticity among its various components. There are many organic mixtures containing aromatic components not sharply distinguishable from others present in degree of aromaticity and the treatment of such grey area mixtures, either for the separation of the aromatics in toto or for further separation of said aromatics into fractions of varying degrees of aromaticity, lies within the purview of my invention. It is more difficult to separate such grey area aromatics into distinct fractions than it is to separate more sharply black and white" aromatics, such as diaromatics and monoaromatics of roughly the same boiling range, but such separations are possible and hence within the scope of my invention. Among the above noted grey area aromatic compounds of intermediate degrees of aromaticity may be mentioned those substituted aromatics containing functional groups of such nature as to have a significant effect of one sort or another on the aromaticity of their unsubstituted counterparts. It will be clear that a wide variety of feed mixtures can be resolved by the method taught herein.

Attention is now directed to the accompanying drawing which schematically illustrates a preferred process for the practice of my invention.

Referring specifically to the drawing, there is shown a continuous countercurrent solvent extraction process employing a solvent of the type described more fully hereafter, such as for example, triethylammonium thiocyanate, the reaction product of stoichiometric quantities of triethylamine and thiocyanic acid. While it is possible to prepare triethylammonium thiocyanate by simply mixing equivalent molar amounts of triethylamine and thiocyanic acid, my preferred way of preparing that material is to mix equimolar proportions of triethylamine and NH SCN and then fractionate off the NH;;:

My reasons for preferring ammonium thiocyanate to thiocyanic acid as a starting material for the preparation of my amine-thiocyanic acid solvent mixtures are the cheapness, greater availability and relative stability of the former by comparison with the latter.

A feed stream containing both aromatic and nonaromatic hydrocarbons of like boiling ranges such as a heavy reformate petroleum fraction containing diaromatics (naphthalene, alkylated naphthalenes, etc.,); monoarornatics (alkyl benzenes, alkyl tetralins, alkyl indanes, etc.,); and nonaromatics (naphthenes and paraffins) is continuously fed into the bottom of a countercurrent solvent extraction column 1 through line 3 as shown. There simultaneously, solvent is recycled into the top of column 1 through line 5, from a source hereinafter disclosed. Solvent extraction column 1 is so designed and the conditions of operation so fixed and controlled as to result in the extraction of substantially all of the aromatic components from the feedstock as it circulates upward in countercurrent contact with the solvent in said column.

As those skilled in the art realized, there are generally four important things to be considered in the practice of countercurrent solvent extraction, namely: (1) the operating temperature; (2) the operating pressures; (3) the number of extraction stages; and (4) the solvent/feed ratio. Careful selection and control of operating conditions in the above four areas is important in order to achieve optimum phase separation, selectivity, and solvent power, all of which have a bearing on product yield and purity.

The operating temperature level is important in solvent extraction operations since in the usual case too low a temperature results in an inordinately high feed viscosity which in turn results in unnecessarily long phase separation periods. Additionally, too low a temperature is undesirable in that it normally has a detrimental effect on the solvent power of selective solvents. On the other hand, excessively high temperatures are undesirable since they usually have the effect of reducing solvent selectivity and tend to make the extract and raflinate phases mutually miscible. Where solvent extraction is carried out at atmospheric pressure, the operating temperatures should preferably not exceed the boiling points of any of the various components present in the system since this would obviously have a deleterious effect on the operation.

Reasons have been given why extremes of temperature in either direction are undesirable in solvent extraction operations. However, the effects of lowering the operating temperature below a certain level are not necessarily all bad since such lowering frequently results in an increase of solvent selectivity. Furthermore, not all of the effects of an excessive elevation of temperature are necessarily deleterious, since such elevation normally brings about a shortening of the time required for phase separation as well as an improvement in the solvent power of selective solvents. It will be apparent from the above-noted considerations that the selection of an optimum temperature range for solvent extraction purposes depends on many factors and entails a balancing of the advantages and disadvantages inherent in various temperature adjustments, taking into consideration the characteristics of the components present in the system, the operating pressures, etc. So too, the selection of optimum operating pressures for solvent extraction operations is subject to the consideration of other factors of an influencing nature. Thus operating pressures can vary from subatmospheric, through atmospheric, to superatmospheric ranges depending upon the peculiarities of the given system.

Returning now to the discussion of the drawing, I have discovered that solvent extraction column 1 is effectively operative on typical systems of the type contemplated when maintained at atmospheric pressure and within an operating temperature range from about 20 to about 150 C.,, the preferred temperature range being from about 30 to about 60 C.

The yield and purity of the rafiinate and extract products, identified infra, from column 1 are, as previously indicated, partially dependent upon the number of extraction stages in said column. It is, as a general rule, true that the greater the number of stages in a solvent extraction column, the greater will be the yield and purity of a product of the column. However, as those skilled in the solvent extraction art will appreciate, the selection of an optimum number of stages is a matter of economics since as the number is increased a point of diminishing returns is reached beyond which the relatively small improvement per additional stage makes it impractical to proceed. I have found that solvent extraction column 1 is operatively effective for use in processes of the contemplated type when it has from about 2 to about 15, and preferably from 7 to 11, stages.

Referring again to the drawing, an extract phase is withdrawn from the bottom of solvent extraction column 1 through line 7 and a raffinate phase is withdrawn from the top of the column through line 9 as shown. When column 1 is designed and operated according to the preferred precepts and conditions set forth above, aromatic hydrocarbon yields of from about to about 99 percent by weight and purities of from about to about 99 percent by weight are attainable in the extract phase. The extract phase contains most of the solvent passing through column 1, in addition to that portion of the feedstock which the solvent has extracted in its travel within the column. The rafiinate phase from column 1, under preferred conditions of operation, typically contains between about 90 and about 99.5 percent, by weight, nonaromatic hydrocarbons, the remainder being solvent.

The raflinate phase from column 1, as the drawing shows, is subject to an alternate choice of disposition. Thus, the raffinate can be withdrawn from the process through valve 11, with valve 13 closed, without further treatment, or it can be circulated to countercurrent solvent extraction column 17 through line 15, as shown on the drawing, for removal of traces of the amine-acid solvent picked up in column 1, using water as the solvent. Ob-

viously, valve 13 is kept open and valve 11 closed in the latter event. Water extraction of the column 1 rafiinate to remove amine-acid solvent therefrom is possible because the solvent, as will be emphasized later, is quite soluble in water.

For ease of illustration, and simplicity of explanation, the process shown on the drawing and described here is depicted as one yielding a recovery of all feedstock aromatics in one pass through column 1. However, it is to be understood that, as explained above, my method of solvent extraction is equally applicable to the separation of aromatics into fractions dilfering as to degree of aromaticity, such as for example the recovery of diaromatics from a feedstock comprising both diaromatic and monoaromatic components. To accomplish this type of separation it is only necessary to assure the proper number of stages in column 1 and the proper operating conditions to accomplish the purpose. The determination of optimum plate plurality and operating conditions to achieve this, or any other result Within the scope of my invention, is a relatively simple matter to those skilled in the solvent extraction art, in the light of the teachings herein, requiring at most a minimum amount of routine experimentation.

Primarily, the achievement of maximum separating etficacy among compounds of varying degrees of aromaticity is a matter of having a sufliciently high number of extraction stages in column 1. In this respect it is pointed out that more stages are required for such selective extraction, all other things being equal, than for mere separation of a mixture into aromatic and nonaromatic fractions.

If it is desired to obtain two or more aromatic fractions from a mixture containing both aromatics of differing aromaticities and nonaromatics, the subject process can by relatively simple modification be made to accommodate this requirement. One Way of accomplishing this is to add one or more additional solvent extraction columns similar to column 1 to the process, with appropriate lines, fittings, and other equipment, where needed, to handle the various streams in the system. For example, if it is desired to separate a diaromatic fraction and a monoaromatic fraction from a feedstock containing diaromatics,

monoaromatics and nonaromatics, this can be done by incorporating an additional solvent extraction column in series with column 1; using column 1 to extract the .diaromatics from the feedstock; and utilizing the additional column to separate the monoaromatics from the nonaromatics in the raffinate phase from column 1.

While the instant method is preferably practiced as a continuous or flow process, and that aspect of operation is stressed in this description, it functions equally well as a batch process provided column 1 is considered to represent suitable apparatus for batch extraction, countercurrent batch extraction, or the like, purposes.

To continue with the detailed description of the process shown on the drawing, in the event the raflinate phase from column 1 is routed to solvent extraction column 17, it is there countercurrently contacted with water which is introduced into said column through line 19. The water extracts substantially all of the solvent contaminant from the rafiinate phase which is then Withdrawn through line 21 to storage or other disposition. The aqueous phase from the extraction operation is withdrawn from column 17 through line 23 as shown and either discarded, recycled for reuse in column 17, or otherwise disposed of. The amount of amine-acid solvent picked up by the water passing through column 17 is so slight as to rule out the practicality, in most cases, of any attempt to recover amine-acid solvent from said aqueous phase. Any other liquid having properties suitable for the purpose may be substituted for the Water employed in column 17, if desired.

The extract phase from solvent extraction column 1 is passed into solvent extraction column 25, through line '7,

wherein the amine-acid solvent (being water soluble, as pointed out supra) is selectively extracted therefrom With water. The extraction of the solvent from the extract phase is not limited to the use of water as the extracting agent and any other material which is a relatively stable liquid under the conditions-of service, and in which the amine-acid solvent is substantially miscible, may be employed for the purpose if desired. It is, of course, obvious that the chosen material must also be substantially immiscible with the hydrocarbon, or hydrocarbons, in the extract phase.

The products from column 25 are an aqueous solution of the amine-acid solvent (aqueous phase) which is withdrawn from the bottom of the column through line 27, and the hydrocarbon product consisting of the aromatic fraction of the feedstock in substantially pure form, which is withdrawn through line 29 to storage or other disposition.

The aqueous solution of solvent from column 25 is passed through line127 to a distillation column 33 in which the water is separated from the solvent, the water passing off through line 31 as an overhead and the solvent Ibeing withdrawn through line 5 as a bottoms product. The water overhead from column 33 is condensed in condenser 35 and then recirculated to solvent extraction column 25 through line 37 as shown on the drawing. The solvent bottoms product from column 33 is recycled through line 5 to solvent extraction column '1.

Column 25 is shown schematically as a countercurrent solvent extraction column and the preferred technique for practicing the subject process is by continuous operation of that unit as well as all other units in the process. However, it is possible to replace column 25 by batch apparatus for solvent extraction if desired. It is also possible to modify the process in other ways obvious to those skilled in the art without substantial alteration of its purpose or accomplishments. One such modification is, for example, the incorporation of purification techniques for the treatment of one or more of the products of separation from solvent extraction column 25 as well as distillation column 33'. Makeup amine-acid solvent is introduced into the system through line 39 and valve 4 1 as needed.

There are a number of things to consider in the selection of solvents for solvent extraction purposes, among which are: (1) the stability of the candidate material under'the conditions (heat, pressure, etc.) of service; (2) the selectivity of said material under service conditions, with respect to the mixture to be extracted; (3) the tendency or lack of such tendency of a material to react chemically with any of the components of systems in which it will be used; (4) its boiling point relative to the boiling points of the components of the mixtures to be separated; (5) its melting point; (6) its corrosiveness towards the materials of construction of the equipment to be employed; (7) its toxicity (which is important from an operational standpoint); (8) its water solubility; and (9) its density relative to the densities of the components of the mixtures to be separated. I have now discovered that mixtures of ammonia and/or amines and cyanic and/ or thiocyanic acids varying widely as to component proportions satisfy all of the requirements inherent in the above points of consideration to qualify as excellent solvents for the selective extraction of aromatic hydrocarbons from organic mixtures of the type previously disclosed. I have further found that, in addition to having an extraordinary selectivity for aromatic hydrocarbons in general, such amine-acid mixtures have relatively greater selectivities for aromatics of relatively greater degrees of aromaticity, thus making the separation of aromatics by degree, as discussed in detail supra, possible by solvent extraction means.

The amine-acid mixtures suitable as selective solvents for use in my invention must, of necessity, be liquid under the service conditions of my solvent extraction method.

The term amine-acid, without further modification, is used throughout this specification to refer to the unique solvent mixtures of this invention. It is, of course, to be understood that the amine-acid ingredient combinations thereby contemplated are those combinations within the scope of this invention as defined and exemplified herein.

The amines of most efiectiveness as solvent ingredients for purposes of this invention are those amines having from about 1 to about 15, and preferably from about 1 to about 9, molecular carbon atoms. The amines may be wholly or predominantly of aromatic, aliphatic or heteroeyclic character, or they may partake, in varying degrees, of the characteristics of compounds in any two, or all three, of those categories. Amines which are otherwise suitable, having substituent groups of a substantially neutral character with respect to other components in the system can be employed for my purpose if desired.

The amine-acid solvents of this invention are not amides but merely, in effect, mixtures of ammonia and/ or amines and acids. Such mixtures are sometimes referred to as salts. However, the term salts has been, to a large extent, avoided herein to diminish the possibility of misunderstanding as to its meaning. Although as indicated above, my class of amine-acid solvents is ex elusive of amides, this poses no particular problem in the selection of suitable amines as solvent ingredients since, insofar as I am aware, no amines form amides with the acids of this invention. Any mixture of eyanie acid, thiocyanic acid or a combination of those acids, and ammonia, any amine, combination of amines, or ammonia-amine combination which is liquid under the service conditions of my solvent extraction method, is a useful solvent for purposes of, and within the scope of, this invention. Primary, secondary and tertiary amines, and mixtures thereof, are all suitable amine ingredients for my solvent mixtures.

Although, for the reason given above, I have deemphasized use of the term salts in connection with my amine-acid solvents. I have found that a stoichiometric proportion of ammonia or amine to acid, which results in a high salt content solvent, is optimum for purposes of this invention. I attribute the superiority of stoiehiometrically balanced solvent mixtures to the fact that cyanic and thioeyanic acids aresubstantially less stable than their ammonium and substituted ammonium salts. While it is preferable not to have large excesses of cyanic or thiocyanic acid present in my solvent mixtures, because of the tendency of those acids toward instability, it is nevertheless within the scope of this invention to employ amine2acid molar ratios other than the stoiehiometric ratio (1:1). In this connection, I have determined that the preferred range of molar ratios is from about 0.5 :1 to about 2:1, although ratios outside of this range are also operative, particularly in those cases in which the amine is present in excess (amines being relatively stable by comparison with eyanic or thiocyanic acid).

A list of tertiary amines useful as solvent ingredients for purposes of this invention appears below. It should be emphasized that this list is merely representative, and not lirnitative of the class of tertiary amines suitable for my purpose.

Triethylamine Trimethylamine Methyldibutylamine Dimethylbutylamine Ethylmethylpropylamine Ethyldipropylamine Diethyloctylamine Diethylpropylamine N,N,N,N-tetramethylethylenediamine N-ethylpyrrolidine N-methylpyrrolidine Examples of secondary amines which can be employed as solvent ingredients for purposes of this invention are given below. Here again, as in the ease of the tertiary amines, the list is exemplary only and not exhaustive.

Diethylamine Ethyl-sec-butylamine Di-isopropylamine Dimethylamine Diethanolamine Morpholine Acetylisobutylamine Piperizine Diphenylamine Monomethylaniline Dibenzylamine Benzylaniline Butylaniline Methylaniline Examples of primary amines suitable as solvent ingredients for purposes of this invention are listed below. The following list is, as in the case of the tertiary and the secondary amines, exemplary only.

Z-aminopropane 2-amino-Z-methylpropane Ethylarnine Methylarnine Aniline n-Propylamine n-Butylamine Sec-butylamine Tert-butylamine Isobutylamine n-Amylamine n-Hexylamine Ethylenediamine Tetramethylenediamine Pentamethylenediamine Ethanolamine Diethylenetriamine Putreseine Cadaverine Aniline Benzylamine fi-Phenylethylamine p-Bromoanaline p-Aminophenyl p-Phenylencdiamine As already indicated, my novel amine-acid solvents are limited acid-wise to the inclusion of cyanic or thiocyanic acid or mixtures of those two acids. One method of preparing the solvents is to simply mix the appropriate amine and acid starting materials by stirring or other means. However, for previously indicated reasons, a preferred method, in many cases, of preparing my solvents is to react a suitable acid derivative with the chosen amine. A suitable acid derivative for such purpose, is one which reacts with the amine in such fashion as to yield an amine-acid product of substantially the same type as that resulting from a straight mixture of the same amine with the acid corresponding to the derivative. Thus, ammonium salts of cyanic and thiocyanic acids (ammonium cyanate and ammonium thiocyanate) are excellent acid derivatives for the purpose since the ammonia by-product is readily eliminated as a gas and does not contaminate the solvent.

Another reason for the excellence of the ammonium salts of cyanic and thiocyanic acid as starting materials for the preparation of my amine-acid solvent mixtures is the fact that those salts are themselves solvents within the scope of this invention. Thus, excesses of ammonium cyanate or ammonium thiocyanate present in my amine- .acid solvents are not present as contaminants but as sol- ,vent of an ammonia-acid rather than an amine-acid type. Solvent mixtures such as the instant one, as Well as mixtures of any of the nitrogen base compounds (ammonia and amines) and acids disclosed herein, are all within the scope of my invention.

As already made clear, my amine-acid solvent must be liquid under the service conditions of my solvent extraction method. This does not mean that the amine-acid solvent must be liquid at standard temperature and pressure conditions although such is frequently the case. It is within the scope of my invention to employ amine and/ ,or acid (or acid salt) starting materials which are solid,

or form solid or semisolid mixtures, at standard conditions, so long as the resulting amine-acid mixtures are liquid under the operating conditions contemplated.

In the preparation of solvent mixtures according to this invention, if theingredients are all liquid at room temperature simple stirring is usually sufficient to effect rapid homogenity of the mix. Where not all of the ingredients are liquid at the temperature of preparation it might be necessary to use more rigorous means of achieving uniformity of mix, such as, for example, kneading or the like. In either event, the application of heat to change the viscosity characteristics of the system can be employed if desired.

As emphasized previously, .the amine-acid mixtures of this invention exhibit extraordinary powers of selectivity toward aromatics when used as solvents in this invention. In addition to being highly selective toward aromatic hydrocarbons, the amine-acid solvents of this invention also exhibit high solvent power, i.e., relatively small quantities of solvent dissolve relatively large quantities of aromatics. In striking illustration of this, I have found that a stoichiometrically proportioned mixture of triethylamine and thiocyanic acid exhibits a solvent power with respect to u-methylnaphtalene, an aromatic hydrocarbon, in the presence of decalin, of 23.8 by comparison with a solvent power of 8.0, under the same circumstances, for 'phenol, a known solvent for aromatic hydrocarbons.

These solvent power values were arrived at by multiplying the difference in a-methylnaphthalene concentration between the solvent-free extract and the feedstock times the percent a-methylnaphthalene recovery in the extract times The trialkylamines are particularly suitable for use as amine ingredients in my novel solvents.

, While the amine-acid mixtures of this invention, and particularly the stoichiometrically balanced trialkylaminethiocyanic or cyanic acid mixtures, are highly selective for aromatics (as will be illustrated in an example to .follow, showing a recovery of 97%, of 93% naphthalene,

from a 40% naphthalene feedstock), I have discovered that the incorporation of a dialkylamine, such as diethylamine, thereinto, results in an increase in product purity, with no sacrifice of recovery, when such mixtures are employed for the d-ifiicult extraction of naphthalene from hydrocarbon mixtures containing, in major proportion, monoaromatics rand nonaromatics. The use of a dialkylamine in conjunction with a trialkylarnine in the manner suggested has an additional advantage in that it normally results in a solvent which is liquid at room temperature, whereas without the dialkylamine the solvent would ordinarily have a melting point higher than room temperature (e.'g., where the ingredients are thiocyanic acid and triet-hylamine, the melting point of the solvent, in its preferred formulation, is about 30 C.).

An important factor in arriving at optimum conditions of operation in solvent extraction processes is the rapidity with which the r-affinate phase separates from the extract phase under various circumstances. Relatively rapid separation of these two bases takes place when using the novel amine-acid solvents of my invention at room temperature. At higher temperatures, phase separation is faster and the solvent power of the amine-acid solvent is greater, but temperature increases have-an adverse effect on selectivity of the solvent towards aromatics. The previously recommended temperature ranges for the practice of my invention (about 20 to about C., preferably from about 30 toabout 60 C.) were determined by taking the aforesaid factors into consideration.

The solvents of this invention are relatively non-toxic at standard, as well as operating, conditions. Also, these solvents are miscible with water, as previously indicated, thus making it an easy matter to recover them from extract phases, as well as raffinate phases, by water extraction, for recycling or other purposes. In addition, the solvents of this invention are unreactive with the feedstock components throughout the temperature range to which they are subjected in service.

The solvent extraction process of my invention can be carried out in various ways, the most common mode of operation comprising the use of a spray, packed, or bubble plate tower wherein the hydrocarbon feed mixture is contacted by the stream of amine-acid solvent flowing, usually countercurrently, therethrough. It is within the scope of my invention to add a minor amount of water, or other inert agent, to my amine-acid solvents where such can be done without colorable modification of those solvent properties necessary to the proper functioning of the process. For example, where'it is proposed to subject .a mixture of hydrocarbons of varying degrees of aromaticity to solvent extraction according to my method, and the hydrocarbons are all extremely soluble in the chosen solvent, the incorporation of a minor amount of water into the solvent, prior to or during the extraction operation, to assure the rapid and distinct formation of two liquid phases is within the spirit and scope of my invention. When water is added for such purpose, the proportion used should normally not exceed about 20% of the weight of the solvent and preferably fall within the range from about 2 to about 5% of the solvent weight.

If desired, my process cm be carried out by distilling the hydrocarbon feed mixture in the presence of an amine-acid solvent, of the type disclosed herein, as an extractive distillation process. In the practice of extractive distillation,.the feed mixture to be separated is distilled in the presence of a selective solvent which has a substantial-1y lower volatility than any of the components of said mixture, as a result of which an overhead enriched in that portion of the feed mixture not selectively extracted is removed, leaving behind a bottoms product which comprises a solution of solvent and selectively extracted material. It is pointed out, however, that a number of my amine-acid solvents tend toward instability at elevated temperatures and therefore when considering the desirability of a particular solvent for an extractive distillation operation attention must be paid to the nature of the material to be separated in order to avoid those tempera- 11 tur'es potentially destructive of the solvent. One possible way of avoiding this difliculty, where the separation of high boiling mixtures is contemplated, is the use of reduced pressures during the operation.

Still another way in which my process can be carried out is to employ an antisolvent in conjunction with the amine-acid solvent, in any manner known to those skilled in the art. The use of such antisolvents in hydrocarbon solvent extraction processes is Well known and need not be considered in detail here. Typical ant-isolvents for purposes of this invention are parafiins such as pentane, heptane, octane, isooctane, and the like; water; etc.

In order to more fully illustrate the invention the following examples are set forth. These examples are to be considered as illustrative only and not limitative of the scope of the invention.

Example I This example illustrates the use of a triethylaminethiocyanic acid mixture as a solvent in the method of my invention.

A 100 gram hydrocarbon feed sample comprising paramns, naphthenes, alkyl indanes, alkyl indenes, alkyl tetralins, and the like, in the amount of 67%, and methylnaphthalene in the amount of 33% was treated with 67 g. of a triethylamine-thiocyanic acid mixture, in which the acid and amine components were present in stoichiometric proportions, having a density of 0.998, in a one stage solvent extraction process. After separation of the phases it was found that the hydrocarbon portion of the extract phase contained approximately 45% of the methylnaphthalene in the feed sample and had a purity of approximately 90%. The solvent was most effective as evidenced by the yield and purity of the methylnaphthalene extract product.

Example II This example serves to illustrate the efiFect of increasing the number of operating stages and the ratio of solvent to feed in my solvent extraction method.

A hydrocarbon dealkylation product containing 40% naphthalene and 60% alkylated monoaromatics and nonaromatics such as naphthenes, alkyl indanes, alkyl indenes, alkyl tetralins, and the like, was subjected to solvent extraction treatment according to the method of this invention utilizing the same solvent as that employed in Example I. Five theoretical operating stages were employed and the solventfeed ratio was 1.25.

An analysis of the extract phase showed a recovery of naphthalene of approximately 97% and a purity of approximately 93%. A comparison of these results with those of Example I, in which the weight ratio of solvent to feed was little more than half that here, and in which only one stage, rather than five, was employed, sharply points up the beneficial effects on recovery and purity, and particularly the former, of an increase in solvent:feed ratio and number of solvent extraction stages.

Example III This example illustrates the use of a triethylaminecyanic acid mixture as a solvent in the method of my in. vention.

A 100 g. hydrocarbon feed sample comprising parafiins, naphthenes, alkyl indanes, alkyl indenes, and alkyl tetralins, in the amount of 67% by weight, and methylnaphthalene in the amount of 33% by weight, is treated with 100 grams of a stoichiometrically proportioned triethylamine-cyanic acid mixture in a l-stage solvent extraction process.

The extract phase from the solvent extraction process is subjected to a water extraction treatment whereby its triethylamiue-cyanic acid solvent portion is selectively extracted by the water. The remaining hydrocarbon portion of the extract phase is enriched in methylnaphthalene.

12 Example IV This is an example of the use of an antisolvent in the practice of my invention.

A single stage solvent extraction operation is carried out at 25 C. by treating a mixture of 14 ml. of dodecane and 6 m1. of methylnaphthalene with 16 m1. of triethylamine-thiocyanic acid solvent (approximately 68% thiocyanic acid) at a weight ratio of solvent to hydrocarbon feed mixture of approximately 1.821. The solvent-hydrocarbon mixture is moderately agitated and a solvent rich extract phase is separated therefrom.

The solvent rich extract phase is washed with pentane, as an antisolvent, to selectively extract dodecane which is dissolved therein as a contaminant. As a result of the washing operation, two phases (a pentane rich phase containing dodecane and a solvent rich phase containing methylnaphthalene) are formed. The two phases are separated and the solvent rich phase is subjected to water extraction treatment to remove the triethylamine-thiocyanic acid solvent therefrom and recover the methylnaphthalene as an extract product.

Example V This example illustrates the use of a tributylaminecyanic acid mixture as a solvent in the method of my invention.

A g. hydrocarbon feed sample comprising paraffins, naphthenes, alkyl indanes, alkyl indenes, and alkyl tetralins, in the amount of 67% by weight, and methylnaphthalene in the amount of 33% by Weight is treated with 100 grams of a stoichiometrically proportioned tributylamine-cyanic acid mixture in a one stage solvent extraction operation. The solvent-hydrocarbon feed mixture is moderately agitated and a solvent rich extract phase is sepanated therefrom.

The solvent rich extract phase is subjected to a water extraction treatment whereby its tributylamine-cyanic acid solvent portion is selectively extracted by the water. The remaining hydrocarbon portion is enriched in methylnaphthalene.

Example VI This example illustrates the use of an aniline-thiocyanic acid mixture as a solvent in the method of my invention.

A 100 g. hydrocarbon feed sample comprising paralfins, naphthenes, alkyl indenes, alkyl indanes and alkyl tetralins in the amount of 67 by weight, and methylnaphthalene in the amount of 33 by weight, is treated with 100 grams of a stoichiometrically proportioned mixture of aniline and thiocyanic acid in a one stage solvent extraction operation. The solvent-hydrocarbon feed mixture is moderately agitated and a solvent rich extract phase is separated therefrom.

The extract phase from the solvent extraction operation is subjected to a water extraction treatment whereby its aniline-thiocyanic acid solvent portion is selectively extracted by the water. The remaining hydrocarbon portion is enriched in methylnaphthalene.

Example VII This example illustrates the use of a Z-aminopropanecyanic acid mixture as a solvent in the method of my invention.

A 100 g. hydrocarbon feed sample comprising parafiins, naphthenes, alkyl indanes, alkyl indenes, and alkyl tetralins, in the amount of 67% by weight, and methyl naphthalene in the amount of 33% by weight is treated with 100 grams of a Z-amino-propane-cyanic acid mixture (in which the acid and amine ingredients are present in stoichiometric proportions) in a one stage solvent extraction operation. The solvent-hydrocarbon feed mixture is moderately agitated and a solvent rich extract phase is separated therefrom.

The extract phase from the solvent extraction operation is subjected to a Water extraction treatment whereby 13 7 its Z-amino-propane-cyanic acid solvent portion is selec tively extracted by the water. The remaining hydrocarbon portion is enriched in methylnaphthalene.

It will be apparent that many modifications of my process can be practiced simply by varying the permissible solvent components, feed materials, and operating techniques within the limits taught herein. All percentage data in the above examples and elsewhere in this disclosure are on a weight basis unless otherwise specified. The present invention is not limited to the use of water extraction for recovery of the hydrocarbon portion of the extract phase, and any other means known to those skilled in the art to accomplish this purpose may be employed if desired. For example, it is possible, in some cases at least, to recover the hydrocarbon portion of the extract phase by distillation techniques. The success of such techniques depends in many instances on the use of reduced pressures to avoid destruction of the solvent, if it is of a type substantially unstable at elevated temperatures.

The preferred solvent: hydrocarbon feedstock weight ratios for purposes of this invention are from about 0.25 :1 to about 3.0:1. However, my process is not limited to this range of ratios and the ratio may vary within wide limits. within the scope of the invention.

The present process is particularly well adapted to preparing feedstocks for dealkylation processes. Thus, a heavy reformate fraction containing alkyl naphthalenes and non-naphthalenic materials can be treated in accordance with the invention to obtain an alkylnaphthalene concentrate which is thereafter dealkylated to form naphthalene by any of the conventional catalytic or thermal dealkylation processes.

I claim:

1. A method of separating hydrocarbon material of greater aromaticity from a mixture thereof with hydrocarbon material of lesser aromaticity which comprises contacting said mixture with a substantially amide-free mixture of a material selected from the group consisting of ammonia and amines and an acid material selected from the group consisting of thiocyanic acid, cyanic acid and mixtures of thiocyanic and cyanic acids.

2. A method of separating hydrocarbon material of greater aromaticity from a mixture thereof with hydrocarbon material of lesser aromaticity but roughly the same boiling point which comprises subjecting said mixture to extractive distillation in the presence of a substantially amide-free mixture of a material selected from the group consisting of ammonia and amines and an acid material selected from the group consisting of thiocyanic acid, cyanic acid and mixtures of thiocyanic and cyanic acids.

3. A method of extracting hydrocarbon material of greater aromaticity from a mixture thereof with hydrocarbon material of lesser aromaticity which comprises contacting said mixture With a solvent comprising a substantially amide-free mixture of a material selected from the group consisting of ammonia and amines and an acid material selected from the group consisting of thiocyanic acid, cyanic acid and mixtures of thiocyanic and cyanic acids, to form an extract phase containing hydrocarbon material rich in said hydrocarbon material of greater aromaticity, and a raifinate phase.

4. A method of extracting hydrocarbon material of greater aromaticity from a mixture thereof with hydrocarbon material of lesser aromaticity which comprises: (1) contacting said mixture with a substantially amide-free mixture of a material selected from the group consisting of ammonia and amines and an acid material selected from the group consisting of thiocyanic acid, cyanic acid and mixtures of thiocyanic and cyanic acids, to form an extract phase containing hydrocarbon material rich in said hydrocarbon material of greater aromaticity, and a raffinate phase and (2) recovering substantially all of the hydrocarbon material of greater aromaticity from said extract phase.

5. The method of claim 4 in which the hydrocarbon material of greater aromaticity is recovered from the ex- 14 t tract phase by extracting the amine-acid solvent from that phase with water.

6. A method of extracting hydrocarbon material of greater aromaticity from a mixture thereof with hydrocarbon material of lesser aromaticity which comprises: (1) contacting said mixture-with a substantially amidefree mixture of a material selected from the group consisting of ammonia and amines and an acid material selected from the group consisting of thiocyanic acid, cyanic acid and mixtures of thiocyanic and cyanic acids, to form an extract phase containing hydrocarbon material rich in said hydrocarbon material of greater aromaticity and a minor amount of said hydrocarbon material of lesser aromaticity, and a raffinate phase; (2) treating the extract phase with an antisolvent to extract said hydrocarbon material of lesser aromaticity therefrom; and (3) recovering substantially all of the hydrocarbon material of greater aromaticity from the extract phase.

7. The method of extracting diaromatic hydrocarbons from a feedstock containing diaromatic, monoaromatic and non-aromatic hydrocarbons comprising: (1) continuously contacting said feedstock, in countercurrent relationship, with a solvent consisting essentially of a mixture of thiocyanic acid and a trialkylamine at a molar ratio of the former to the latter of from about 120.5 to about 1:2, whereby an extract phase rich in said solvent and containing a portion of the feedstock enriched in diaromatic hydrocarbons and a raflinate phase containing substantially all of the remaining portion of the feedstockand a minor amount of said solvent, are obtained; (2) subjecting the extract phase from step 1 to water extraction to form an aqueous phase containing water and substantially all of the solvent in said extract phase, and a hydrocarbon phase containing substantially all of the diaromatic hydrocarbons from said extract phase; (3) subjecting the aqueous phase from step (2) to fractional distillation to form an overhead product of substantially pure Water and a bottoms product of substantially pure solvent; 4) recycling the solvent bottoms product from step (3) to solvent extraction step (1); (5) condensing the overhead water product from step (3); and (6) recycling the condensed water from step (5) to water extraction step (3).

8. The method of claim 7 in which the trialkylamine is triethylamine.

9. The method of claim 7 in which the acid and amine ingredients of the solvent are present in stoichiometric proportions.

10. The method of claim 7 in which the raflinate phase from step 1 is subjected to water extraction treatment to remove substantially all of the solvent therefrom and to obtain a substantially solvent free raflinate hydrocarbon product.

11. The method of claim 7 in which step 1 is conducted at atmospheric pressure and at a temperature Within the range from about 20 to about C.

12. The method of claim 7 in which the ratio of solvent to hydrocarbon feedstock in step 1 is from about 0.25 to about 3.0 parts by weight of the former to about 1 part by weight of the latter.

13. The method of extracting diaromatic hydrocarbon material from a mixture comprising said diaromatic hydrocarbon material and monoaromatic hydrocarbon material which comprises: (1) contacting said mixture with a mixture of a tertiary amine and thiocyanic acid, whereby an extract phase containing hydrocarbon material rich in said diaromatic hydrocarbon material and a rafiinate phase, are formed and (2) subjecting said extract phase to water extraction treatment whereby substantially all of the amine-acid mixture is removed therefrom leaving substantially all of the diaromatic hydrocarbon material as the extract product of the process.

14. A method of extracting hydrocarbon material of greater aromaticity from a mixture thereof with hydrocarbon material of lesser aromaticity which comprises: 1) contacting said mixture with a solvent comprising a substantially amide-free mixture of a material selected from the group consisting of ammonia and amines and an acid material selected from the group consisting of thiocyanic acid, cyanic acid and mixtures of thiocyanic and cyanic acids, and containing a minor amount of water, to form an extract phase containing hydrocarbon material rich in said hydrocarbon material of greater aromaticity and a raffinate phase.

15. The method of claim 14 in Which the water is present in the solvent in an amount from about 2 to about 5% by weight thereof.

16. In the method of extracting aromatic hydrocarbon material from a mixture thereof with nonaromatic hydrocarbon material which comprises contacting said mixture with a solvent for the aromatic hydrocarbon material to form an extract phase containing hydrocarbon material rich in said aromatic hydrocarbon material, and a rafiinate phase; the improvement which comprises employing a substantially amide-free mixture of -a material selected from the group consisting of ammonia and amines and an acid material selected from the group consisting of thiocyanic acid, cyanic acid and mixtures of thiocyanic and cyanic acids, as the solvent for the aromatic hydrocarbon material.

17. The improvement of claim 16 in which the solvent for the aromatic hydrocarbon material is a mixture of a.

tertiary amine and thiocyanic acid.

18. The improvement of claim 16 in which the solvent for the aromatic hydrocarbon material is a mixture of a trialkylamine and thiocyanic acid.

19. The improvement of claim 16 in which the solvent for the aromatic hydrocarbon material is a stoichiometrically proportioned mixture of triethylamine and thiocyanic acid.

20. A method of extracting naphthalene and alkylnaphthalenes from a feedstock containing these compounds and close-boiling mono-aromatic hydrocarbons comprising: (1) continuously contacting said feedstock, in countercurrent relationship, with a solvent consisting of a stoichiometrically proportioned mixture of thiocyanic acid and triethylamine, whereby an extract phase rich in said solvent and containing a portion of the feedstock enriched in naphthalene and alkylnaphthalenes, and a rafiinate phase containing susbtantially all of the remaining portion of the feedstock and a minor amount of said solvent, are obtained; (2) subjecting the extract phase from step 1 to water extraction to form an aqueous phase containing water and substantially all of the solvent in said extract phase, and a hydrocarbon phase containing substantially all of the naphthalene and alkylnaphthalenes from said extract phase; (3) subjecting the aqueous phase from step 2 to fractional distillation to form an overhead product of substantially pure water and a bottoms product of substantially pure solvent; (4) recycling the solvent bottoms product from step 3 to solvent extraction step 1; (5) condensing the overhead water product from step 3; and (6) recycling the condensed water from step 5 to water extraction step 3.

No references cited. 

1. A METHOD OF SEPARATING HYDROCARBON MATERIAL OF GREATER AROMATICITY FROM A MIXTURE THEREOF WITH HYDROCARBON MATERIAL OF LESS AROMATICITY WHICH COMPRISES CONTACTING SAID MIXTURE WITH A SUBSTANTIALLY AMIDE-FREE MIXTURE OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF AMMONIA AND AMINES AND AN ACID MATERIAL SELECTED FROM THE GROUP CONSISTING OF THIOCYANIC ACID, CYANIC ACID AND MIXTURES OF THIOCYANIC AND CYANIC ACIDS. 